1. Field of the Invention
This invention relates to techniques for obtaining nuclear magnetic resonance (NMR) information from an object. More specifically, this invention relates to techniques for encoding information into the phase of a received NMR signal.
2. Description of the Prior Art
NMR information about an object is ordinarily obtained by receiving a single time-varying radio frequency (rf) signal. In imaging, for example, this time-varying rf signal must provide the necessary information about the object being imaged, such as a human body. Therefore, the characteristics of this time-varying signal must somehow be modulated by the atomic nuclei in the object to provide information about those nuclei. For example, in Fourier imaging, the distribution of nuclei of a particular element is typically encoded into the phase and amplitude of the received time-varying signal. Various techniques have been developed for enabling the nuclei to encode information into the received signal.
The most common use of phase encoding in NMR imaging is to encode information about the spatial location of atomic nuclei having a particular resonance. Such nuclei are initially excited into a resonant state from which they decay according to an appropriate time constant. While in the excited state, the nuclei may be phase encoded by tbe application of a relatively low frequency pulsed magnetic field gradient. The effect of the low frequency gradient is to change the phase of the decay, but because the gradient varies across a spatial dimension, the amount of phase change varies according to the position of the nuclei, so that the resulting phases of the nuclei provide the desired spatial information. Several variations of this technique have been used, including the application of a range of field gradients having various amplitudes, and the application of a series of field gradient pulses during a series of echoing NMR signals to produce increasing amounts of phase encoding. Each of these variations can be used to obtain a two or three dimensional data set which may be Fourier transformed into a two or three dimensional spatial distribution of the excited nuclei. If the NMR signals themselves are received in the absence of any applied field gradient, but after phase encoding in all resolved spatial dimensions, spectroscopic frequency information about the nuclei at each spatial location will be received, indicating chemical shift information or field inhomogeneity information about the resonant atoms at that location.
Another technique, referred to herein as the echo-time encoded method, phase encodes spectroscopic information by changing the timing of one or more refocusing pulses in a pulse and gradient switching sequence which produces a series of spin echoes. The sequence also includes the application of an observation gradient during the echoing NMR signals, so that the refocusing of the nuclear spins is determined by the combination of the refocusing pulse and the observation gradient. A mismatch of the timing of these signals results in a phase change of the received NMR signal which evolves during the observation gradient and which depends on the frequency difference between the spin resonance of the nucleus and the reference frequency absent applied field gradients. This technique makes it possible to separate two chemically shifted systems such as lipids and water having the same resonant nucleus, in this case protons, but different resonant frequencies, and successive applications of the mistimed refocusing pulses make it possible to obtain more detailed spectral discrimination.
Other techniques of encoding use rf pulses with spectra which include discrete frequency bands, but accurate generation of such spectra is difficult in practice. Also, continuous coverage of the complete range of resonant frequencies of an object requires more than one measurement, due to the gaps in the spectra.
A number of techniques have also been developed for amplitude encoding of information, some of which are discussed below. Amplitude encoding, however, suffers a loss of sensitivity compared with phase encoding, so that phase encoding is often preferable.
Most of the above-described techniques of phase encoding depend heavily upon the application of pulsed magnetic gradient fields. These gradient fields are typically applied by gradient coils which have a number of problems, including audio frequency vibrations, high power supply requirements, and heavy duty switching requirements. It would be advantageous to provide phase encoding techniques which could be used without gradient fields. More generally, it would be advantageous to provide phase encoding techniques which would produce a continuous phase variation across the entire range of resonant frequencies using an rf pulse, providing greater flexibility than has previously been available.